Silicic Acid Leakage Research Articles

Diatom Physiology Controls Silicic Acid Leakage in Response to Iron Fertilization

We explore how the iron dependence of the Si:P uptake ratio RSi:P of diatoms controls the response of the global silicon cycle and phytoplankton community structure to Southern Ocean iron fertilization. We use a data‐constrained model of the coupled Si‐P‐Fe cycles that features a mechanistic representation of nutrient colimitations for three phytoplankton classes and that is embedded in a data‐assimilated global ocean circulation model. We consider three parameterizations of the iron dependence of RSi:P, all of which are consistent with the available field data and allow equally good fits to the observed nutrient climatology but result in very different responses to iron fertilization: Depending on how sharply RSi:P decreases with increasing iron concentration, iron fertilization can either cause enhanced silicic acid leakage from the Southern Ocean or strengthened Southern Ocean silicon trapping. Enhanced silicic acid leakage occurs if decreases in RSi:P win over increases in diatom growth, while the converse causes strengthened Southern Ocean silicon trapping. Silicic acid leakage drives a floristic shift in favor of diatoms in the subtropical gyres and stimulates increased low‐latitude opal export. The diatom contribution to global phosphorus export increases, but the lower diatom silicon requirement under iron‐replete conditions reduces the global opal export. Regardless of RSi:P parameterization, the global response of the biological phosphorus and silicon pumps is dominated by the Southern Ocean. The Si isotope signature of opal flux becomes systematically lighter with increasing iron‐induced silicic acid leakage, consistent with sediment records from iron‐rich glacial periods.

Glacial valve size variation of the Southern Ocean diatom Fragilariopsis kerguelensis preserved in the Benguela Upwelling System, southeastern Atlantic

The valve size variations of the Southern Ocean (SO) diatom Fragilariopsis kerguelensis from the sediments of Antarctic Polar Front and Subantarctic Front were shown to be strongly correlated with iron availability at the glacial-interglacial timescale. However, recent evidence from the subtropical region of the SO has suggested sea surface temperature to be a key controlling factor by mediating silica uptake other than the iron availability. We present here the first record of F. kerguelensis size variation in core GeoB3606-1 collected in the Benguela Upwelling System (BUS), SE Atlantic (25°28′S, 13°05′E) where F. kerguelensis valves were preserved during the glacial period (68–30 ka) as a consequence of silicic acid leakage from polar to mid-latitudes. Our study presents the first record of diatom morphology variations of F. kerguelensis from a mid-latitude core location to test the previous hypotheses proposed for SO. Our results demonstrate that F. kerguelensis valves at GeoB3606-1 were almost half of the mean size of specimens compared to the SO downcore records. Further, F. kerguelensis valves were smaller and more abundant during early Marine Isotope Stage (MIS) 3 and conversely larger and less abundant during MIS4 and late MIS3. However, such patterns are not supported by the linear regression analyses suggesting that iron availability could have mediated the size changes of F. kerguelensis rather than the species productivity during 68–30 ka. These findings agree with the recently gained knowledge on the life cycle patterns of F. kerguelensis related to reproduction strategies, cell size range, and formation of initial cells.

A Review of the Stable Isotope Bio-geochemistry of the Global Silicon Cycle and Its Associated Trace Elements

Silicon (Si) is the second most abundant element in the Earth’s crust and is an important nutrient in the ocean. The global Si cycle plays a critical role in regulating primary productivity and carbon cycling on the continents and in the oceans. Development of the analytical tools used to study the sources, sinks, and fluxes of the global Si cycle (e.g., elemental and stable isotope ratio data for Ge, Si, Zn, etc.) have recently led to major advances in our understanding of the mechanisms and processes that constrain the cycling of Si in the modern environment and in the past. Here, we provide background on the geochemical tools that are available for studying the Si cycle and highlight our current understanding of the marine, freshwater and terrestrial systems. We place emphasis on the geochemistry (e.g., Al/Si, Ge/Si, Zn/Si, δ13 C, δ15 N, δ18 O, δ30 Si) of dissolved and biogenic Si, present case studies, such as the Silicic Acid Leakage Hypothesis, and discuss challenges associated with the development of these environmental proxies for the global Si cycle. We also discuss how each system within the global Si cycle might change over time (i.e., sources, sinks, and processes) and the potential technical and conceptual limitations that need to be considered for future studies.

A record of Pleistocene diatom preservation from the Amundsen Sea, West Antarctica with possible implications on silica leakage

article i nfo Accumulation of fossil diatoms in marine sediments is variably affected by the production and dissolution of diatom frustules across space and time. No quantitative proxy exists for estimating the amount of silica lost to dissolution in Southern Ocean sediment cores. To address this gap, cultured valves of Fragilariopsis kerguelensis, the dominant planktonic diatom in the Pleistocene to recent Southern Ocean, were dissolved progressively under controlled laboratory conditions. Expansion, and eventual conjoining, of the areolae of F. kerguelensis advances through several phases, which are presented herein as a measure of diatom dissolution. In order to evaluate the validity of laboratory methods, this proxy is applied to sediments from piston core PS58/254 from the Amundsen Sea. Dissolution-induced degradation morphologies from laboratory experiments are seen to be identical to those preserved naturally in sediments, thus suggesting the laboratory conditions accurately reflect the natural process of dissolution. These resultsrepresentthe firstapplication of afullyquantitativeproxyfordiatomdissolution toSouthernOcean sediments, revealing that meaningful paleoceanographic data can be generated via taphonomic analysis of diatom valves. Intensity of diatom dissolution in core PS58/254 does not correspond to the relative abundance of any diatom species, clay mineralogy, or linear sedimentation rates. However, a correlation does exist between glacial cyclicity and diatom dissolution, indicating that variation in average wall thickness and susceptibility to dissolution reflects primary conditions during formation of the silica wall, transit through the water column, and eventual deposition. Higher overall silica dissolution is observed during each glacial period and enhanced silica preservation during interglacials. This result is consistent with the predictions of the silicic acid leakage hypothesis (SALH), whereby reduced rates of Si usage in the Southern Ocean lead to shifts in marine primary productivity in the tropics from coccolithophoriddominance duringinterglacialstodiatomdominanceduringglacials.Thiswouldfuela decrease in atmospheric pCO2duringglacials. Silica acidleakage isdrivenbya decreaseinthe Si:Nratioofdiatoms,caused by increased iron input to glacial-aged Southern Ocean waters. This resulted in more lightly silicified diatom valves in the Southern Ocean, which dissolved more quickly in the upper water column, thereby maintaining the high silica content of glacial-aged SO waters, and allowing for export of Si to upwelling zones in the lower latitudes.

Seasonal evolution of net and regenerated silica production around a natural Fe-fertilized area in the Southern Ocean estimated with Si isotopic approaches

Abstract. A massive diatom bloom is observed each year in the surface waters of the naturally Fe-fertilized Kerguelen Plateau (Southern Ocean). We measured biogenic silica production and dissolution fluxes (ρSi and ρDiss, respectively) in the mixed layer in the vicinity of the Kerguelen Plateau during austral spring 2011 (KEOPS-2 cruise). We compare results from a high-nutrient low-chlorophyll reference station and stations with different degrees of iron enrichment and bloom conditions. Above the plateau biogenic ρSi are among the highest reported so far in the Southern Ocean (up to 47.9 mmol m−2 d−1). Although significant (10.2 mmol m−2 d−1 on average), ρDiss were generally much lower than production rates. Uptake ratios (ρSi : ρC and ρSi : ρN) confirm that diatoms strongly dominate primary production in this area. At the bloom onset, decreasing dissolution-to-production ratios (D : P) indicate that the remineralization of silica could sustain most of the low silicon uptake and that the system progressively shifts toward a silica production regime which must be mainly supported by new source of silicic acid. Moreover, by comparing results from the two KEOPS expeditions (spring 2011 and summer 2005), we suggest that there is a seasonal evolution of the processes decoupling Si and N cycles in the area. Indeed, the consumption of H4SiO4 standing stocks occurs only during the growing stage of the bloom when strong net silica production is observed, contributing to higher H4SiO4 depletion relative to NO3−. Then, the decoupling of H4SiO4 and NO3− is mainly controlled by the more efficient nitrogen recycling relative to Si. Gross Si : N uptake ratios were higher in the Fe-rich regions compared to the high-nutrient low-chlorophyll (HNLC) area, likely due to different diatom communities. This suggests that the diatom responses to natural Fe fertilization are more complex than previously thought, and that natural iron fertilization over long timescales does not necessarily decrease Si : N uptake ratios as suggested by the silicic acid leakage hypothesis. Finally, we propose the first seasonal estimate of the Si biogeochemical budget above the Kerguelen Plateau based on direct measurements. This study points out that naturally iron-fertilized areas of the Southern Ocean could sustain very high regimes of biogenic silica production, similar to those observed in highly productive upwelling systems.

Southern-ocean and glaciogenic nutrients control diatom export production on the Chile margin

Biogenic particle flux was reconstructed using 230-Thorium normalization at two sites on the southern Chile margin. ODP Site 1233 at 41°S, 838 m depth, is at the southern limit of the Peru–Chile upwelling system, where the northern extent of the Antarctic Circumpolar Current impinges on the South American continental margin. ODP Site 1234, at 36°S, 1014 m depth, is located within the core of the coastal upwelling system near the mouths of the Bio Bio and Itata Rivers. At 41°S, opal, lithogenic and carbonate fluxes are greatest during the Last Glacial interval (26–20 ka), carbonate has a secondary peak during the mid Holocene (∼8 ka) and organic carbon fluxes increase slightly from 17 ka to the present. At 36°S, large lithogenic fluxes are observed both during the Last Glacial interval and the Holocene, and a maximum in organic carbon flux is observed during the late Holocene (∼5 ka) without an accompanying peak in opal flux. These reconstructed fluxes at 36°S and 41°S fit within a larger latitudinal pattern of a poleward increase in the magnitude of opal flux during the glacial period. The pattern of normalized opal flux, opal mass accumulation rate and opal:carbonate ratios is consistent with either i) enhanced supply of Si from the Southern Ocean, as proposed by the Silicic Acid Leakage Hypothesis or ii) enhanced Si and Fe delivery from land, driven by glacial erosion. The pattern of reconstructed export production supports our view that the appearance of more reducing conditions in the sediments upon deglaciation was most likely driven by decreased ventilation, rather than increased local productivity.

Different mechanisms of silicic acid leakage and their biogeochemical consequences

In the modern ocean, silicic acid is effectively trapped in the Southern Ocean. According to the silicic acid leakage hypothesis, a loosening of this trapping may have contributed to low atmospheric CO2 during glacial times. Using a model with dynamical feedbacks in ocean physics and biology, we explore three distinct mechanisms to loosen the trapping and trigger silicic acid leakage from the Southern Ocean: sea ice expansion, weaker winds, and iron addition. The basic idea of the iron addition mechanism was previously explored using a simple box model. Here we confirm the main results of the earlier work and demonstrate further that sea ice expansion and weaker southern westerlies can also trigger silicic acid leakage. The three mechanisms are not mutually exclusive, and their biogeochemical consequences are dissimilar in terms of the spatial patterns of Si:N uptake and the relative changes in opal and particulate organic carbon fluxes both in and outside the Southern Ocean. While it is not entirely clear how sea ice, winds, and iron deposition were different during the last glacial period compared to today, we examine the synergistic effects of these three triggers in one simulation. In this simulation, the combination of sea ice and iron reproduces the north-south dipole of productivity recorded in export production proxies, but effects of the iron perturbation seem to dominate the overall biogeochemical response.

Diatom traits regulate Southern Ocean silica leakage

Ocean circulation is a remarkable interconnecting conduit, such that biological processes occurring in the remote Southern Ocean can influence the regulation of productivity in Northern Hemisphere waters. Marine phytoplankton, despite their small size, play a disproportionately important role in setting the stoichiometric relationship between elements such as nitrogen, phosphorus, carbon and silicon in the global ocean (1, 2). In turn this ecological stoichiometry helps to set the ultimate limiting nutrient(s) for primary productivity (2, 3). To date, investigation of stoichiometric effects has mainly been centered on low latitude waters (3, 4). However, the Southern Ocean plays a fundamental role in setting the productivity of distant waters, such as the Equatorial Pacific (5), by controlling the leakage of waters with high Silicic acid:Nitrate ratios—the so-called silicic acid leakage hypothesis (SALH) (6, 7). In PNAS, Assmy et al. (8) provide unprecedented detail of how the ecological traits of different polar diatom species contribute to the regulation of ocean nutrient stoichiometry. Thus, diatom floristics help to control the leakage of silicic acid, relative to that of nitrate, into the global ocean, which sets the magnitude of Northern Hemisphere diatom productivity, export, and hence carbon sequestration.

Potential role of giant marine diatoms in sequestration of atmospheric CO2 during the Last Glacial Maximum: δ13C evidence from laminated Ethmodiscus rex mats in tropical West Pacific

Giant marine diatoms, blooming or aggregating within deep chlorophyll maxima under stratified conditions, can generate substantial production and a large export flux of organic carbon from surface waters. However, their role in regulating glacial-interglacial variation in atmospheric pCO2 remains unclear. Here, we report the organic carbon isotopic compositions of Ethmodiscus rex diatoms (δ13CE. rex) and bulk sediments (δ13Corg) from a sediment core in the eastern Philippine Sea dated to ~19.5–31.0kyr B.P. and consisting of (from youngest to oldest) (1) laminated E. rex diatom mats (LDM), (2) diatomaceous clay (DC), and (3) pelagic clay (PC). Our results suggest that δ13CE. rex provides a better record of palaeoceanographic processes during LDM and DC deposition than δ13Corg because of reduced differential vital effects in near-monospecific E. rex fractions. We used the isotopic composition of the coarse E. rex fraction (δ13CE. rex (>154μm)) to calculate the CO2 partial pressure of eastern Philippine Sea surface waters (pCO2-sw) during the Last Glacial Maximum (LGM). Our pCO2-sw records suggest that the eastern Philippine Sea switched from being a strong CO2 source during DC deposition to a weak CO2 sink during LDM deposition. The role of the eastern Philippine Sea as a CO2 sink during the LGM was promoted by elevated primary production and, to a lesser extent, intensified water-column stratification. These observations highlight the potential role of giant marine diatoms in the sequestration of atmospheric CO2 during the LGM and, hence, support changes in biogenic silica fluxes as a potential cause of lower glacial atmospheric CO2. Our findings are consistent with an eolian source of silica, as proposed by the ‘silica hypothesis’ and the ‘silicon-induced alkalinity pump hypothesis’ but not by the ‘silicic acid leakage hypothesis.’

Evidence of silicic acid leakage to the tropical Atlantic via Antarctic Intermediate Water during Marine Isotope Stage 4

AbstractAntarctic Intermediate Water (AAIW) and Subantarctic Mode Water (SAMW) are the main conduits for the supply of dissolved silicon (silicic acid) from the deep Southern Ocean (SO) to the low‐latitude surface ocean and therefore have an important control on low‐latitude diatom productivity. Enhanced supply of silicic acid by AAIW (and SAMW) during glacial periods may have enabled tropical diatoms to outcompete carbonate‐producing phytoplankton, decreasing the relative export of inorganic to organic carbon to the deep ocean and lowering atmospheric pCO2. This mechanism is known as the “silicic acid leakage hypothesis” (SALH). Here we present records of neodymium and silicon isotopes from the western tropical Atlantic that provide the first direct evidence of increased silicic acid leakage from the Southern Ocean to the tropical Atlantic within AAIW during glacial Marine Isotope Stage 4 (~60–70 ka). This leakage was approximately coeval with enhanced diatom export in the NW Atlantic and across the eastern equatorial Atlantic and provides support for the SALH as a contributor to CO2 drawdown during full glacial development.

Millennial variability and long‐term changes of the diatom production in the eastern equatorial Pacific during the last glacial cycle

The modern eastern equatorial Pacific (EEP) is a major natural source for atmospheric carbon dioxide and is thought to be connected to high‐latitude ocean dynamics by oceanic teleconnections on glacial‐interglacial timescales. A wealth of sedimentary records aiming at reconstructing last Quaternary changes in primary productivity and nutrient utilization have been devoted to understanding those linkages between the EEP and other distant oceanic areas. Most of these records are, however, clustered in the pelagic EEP cold tongue, with comparatively little attention devoted to coastal areas. Here we present downcore measurements of the composition and concentration of the diatom assemblage together with opal (biogenic silica) concentration at site MD02‐2529 recovered in the coastal Panama Basin. Piston core MD02‐2529, collected in an area affected by a multitude of processes, provides evidence for strong variations in diatom production at the millennial timescale during the last glacial cycle. The maxima in total diatom concentration occurred during the early marine isotopic stage (MIS) 4 as well as during the MIS 4/3 transition and MIS 3. Rapid changes in diatom concentrations during the MIS 3 mimics Bond cycles as independently recorded by the sea surface salinity estimation derived from planktonic foraminifera from the same core. Such patterns indicate a clear linkage between diatom production in the coastal EEP and rapid climate changes in the high‐latitude North Atlantic. In parallel, the long‐term succession of the diatom community from coastal diatoms, predominantly thriving during MIS 5 and 4, toward pelagic diatoms, dominant during MIS 3 and 2, points to a long‐term change in the surface hydrology. During Heinrich events, diatoms strongly reduced their production, probably because of enhanced stratification in the upper water column. After the last glacial maximum, diatom production and valve preservation strongly decreased in response to the advection of nutrient‐depleted (H2SiO4), warmer water masses. Our high‐resolution record highlights how regional climatic processes can modulate rapid changes in siliceous primary production as triggered by wind‐induced local upwelling, indicating that millennial climatic variability can overtake other prominent hydrological processes such as those related to silicic acid leakage.

High-resolution opal records from the eastern tropical Pacific provide evidence for silicic acid leakage from HNLC regions during glacial periods

A shift from carbonate- to silica-dominated primary production could significantly affect the oceanic carbon cycle via changes in the particulate carbon rain-rate ratio ( C organic:C inorganic fluxes). An increase in C rain rate ratio has been invoked to explain lower glacial pCO 2; however, firm evidence of an ecological shift towards silica-dominated productivity during the last glacial period is lacking. Here, we present new high-resolution reconstructions of biogenic silica and total production over the past 40,000 yr BP in 3 cores from the eastern tropical North Pacific (ETNP) off Mexico and Nicaragua. These records reveal a clear regional pattern of higher siliceous productivity with higher opal accumulation during the last glacial period compared to interglacial times. Higher Si:C and Si:N ratios of glacial sediments in these records suggest a net increase in siliceous production over total production. We attribute this to the additional supply of silicic acid to the ETNP margins favouring diatoms over other non-siliceous algae. This suggestion for increased supply of Si during glacial periods is consistent with the proposed large-scale redistribution of excess silicic acid from High Nitrate Low Chlorophyll (HNLC) regions like the eastern equatorial Pacific (EEP) and the Southern Ocean by the Silicic Acid Leakage Hypothesis (SALH). In these HNLC regions, the Si-isotope composition of diatom frustules (δ 30Si) has provided evidence for the generation of surplus of silicic acid during diatom growth under conditions of higher Fe availability during glacial periods. We suggest that silicic acid leakage from the HNLC regions to the adjoining oceans may have increased the carbon rain rate ratio and ultimately, contributed to the decrease in glacial atmospheric pCO 2.

Eastern Equatorial Pacific productivity and related-CO 2 changes since the last glacial period

Understanding oceanic processes, both physical and biological, that control atmospheric CO(2) is vital for predicting their influence during the past and into the future. The Eastern Equatorial Pacific (EEP) is thought to have exerted a strong control over glacial/interglacial CO(2) variations through its link to circulation and nutrient-related changes in the Southern Ocean, the primary region of the world oceans where CO(2)-enriched deep water is upwelled to the surface ocean and comes into contact with the atmosphere. Here we present a multiproxy record of surface ocean productivity, dust inputs, and thermocline conditions for the EEP over the last 40,000 y. This allows us to detect changes in phytoplankton productivity and composition associated with increases in equatorial upwelling intensity and influence of Si-rich waters of sub-Antarctic origin. Our evidence indicates that diatoms outcompeted coccolithophores at times when the influence of Si-rich Southern Ocean intermediate waters was greatest. This shift from calcareous to noncalcareous phytoplankton would cause a lowering in atmospheric CO(2) through a reduced carbonate pump, as hypothesized by the Silicic Acid Leakage Hypothesis. However, this change does not seem to have been crucial in controlling atmospheric CO(2), as it took place during the deglaciation, when atmospheric CO(2) concentrations had already started to rise. Instead, the concomitant intensification of Antarctic upwelling brought large quantities of deep CO(2)-rich waters to the ocean surface. This process very likely dominated any biologically mediated CO(2) sequestration and probably accounts for most of the deglacial rise in atmospheric CO(2).

Sedimentary opal records in the eastern equatorial Pacific: It is not all about leakage

The clear predictions of the silicic acid leakage hypothesis (SALH) resulted in a number of studies of downcore opal records from the tropical Pacific. The original SALH predicts that unused silicic acid, due to Fe‐driven changes in Si versus N limitation, escaped from the glacial Southern Ocean to equatorial upwelling regimes where it enhanced diatom productivity, thereby decreasing coccolith growth and lowering atmospheric CO2. In contrast to SALH predictions, however, sedimentary records from the eastern equatorial Pacific (EEP) do not show enhanced opal burial during the Last Glacial Maximum (LGM) but higher rates of opal burial during the deglaciation and marine isotopic stage 3 (MIS3). The peak in opal productivity during the deglaciation has been attributed to increased supply of nutrient‐rich waters driven by stronger upwelling of deep water in the Southern Ocean at the end of last glacial period. The large peak in opal burial observed in a number of EEP cores during MIS3 was interpreted as evidence for Si leakage when Southern Ocean diatom productivity was limited by both low dust flux and extended sea ice. On the other hand, the paradoxical LGM decline in opal accumulation in the EEP was explained by enhanced dust input that lowered the diatom Si:C uptake ratio. Here we use a combination of molecular fingerprints of algal productivity and radioisotope tracers of sedimentation to revisit opal burial in the EEP, in particular during the MIS3 “opal peak.” An increase in algal productivity is not supported by the sedimentary concentration of brassicasterol, an organic molecule commonly found in diatoms, or by the ratio of (231Pa/230Th)xs,0, a proxy for opal export production. We therefore conclude that the large peak in opal burial during MIS3 reflects enhanced preservation of diatoms. Building on mechanisms invoked in previous studies, we hypothesize that opal burial in the EEP is controlled both by the physiological response of diatoms to low‐latitude Fe inputs and by the high‐latitude processes leading to silicic acid leakage.

Comparing glacial and Holocene opal fluxes in the Pacific sector of the Southern Ocean

The silicic acid leakage hypothesis (SALH) predicts that during glacial periods excess silicic acid was transported from the Southern Ocean to lower latitudes, which favored diatom production over coccolithophorid production and caused a drawdown of atmospheric CO2. Downcore records of 230Th‐normalized opal (biogenic silica) fluxes from 31 cores in the Pacific sector of the Southern Ocean were used to compare diatom productivity during the last glacial period to that of the Holocene and to examine the evidence for increased glacial Si export to the tropics. Average glacial opal fluxes south of the modern Antarctic Polar Front (APF) were less than during the Holocene, while average glacial opal fluxes north of the APF were greater than during the Holocene. However, the magnitude of the increase north of the APF was not enough to offset decreased fluxes to the south, resulting in a decrease in opal burial in the Pacific sector of the Southern Ocean during the last glacial period, equivalent to approximately 15 Gt opal ka−1. This is consistent with the work of Chase et al. (2003a), and satisfies the primary requirement of the SALH, assuming that the upwelled supply of Si was approximately equivalent during the Holocene and the glacial period. However, previous results from the equatorial oceans are inconsistent with the other predictions of the SALH, namely that either the Corg:CaCO3 ratio or the rate of opal burial should have increased during glacial periods. We compare the magnitudes of changes in the Southern Ocean and the tropics and suggest that Si escaping the glacial Southern Ocean must have had an alternate destination, possibly the continental margins. There is currently insufficient data to test this hypothesis, but the existence of this sink and its potential impact on glacial pCO2 remain interesting topics for future study.

A corollary to the silicic acid leakage hypothesis

The silicic acid leakage hypothesis (SALH) attempts to explain part of the large and regular atmospheric CO2 changes over the last glacial‐interglacial cycles. It calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low‐latitude surface waters, driven by a “leakage” of high‐Si:N waters from the Southern Ocean. Recent studies that present low opal accumulation rates from the glacial eastern equatorial Pacific have challenged SALH. In a corollary to SALH, we argue that the key to SALH is the dominance of diatoms over coccolithophorids, and this does not depend on the magnitude of diatom production per se. In support of our claim, we show in a numerical model that atmospheric CO2 can be lowered with even a reduced absolute flux of silicic acid leakage, provided that Si:N in the leakage is elevated and that the excess Si can be used by diatoms to shift the floral composition in their favor.

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

The Silicic Acid Leakage Hypothesis (SALH) suggests that, during glacial periods, excess silicic acid was transported from the Southern Ocean to lower latitudes, which favored diatom production over coccolithophorid production and caused a drawdown of atmospheric CO2. Downcore records of 230Th‐normalized opal fluxes and 231Pa/230Th ratios from seven equatorial Atlantic cores were used to reconstruct diatom productivity over the past 30 ka (where a is years) and to test the SALH. Downcore records of 231Pa/230Th ratios and opal fluxes are highly correlated, suggesting that they constitute a production‐based record of opal flux. Opal flux records support the SALH in that glacial opal burial exceeded Holocene burial by 1.8 Gt opal/ka in the area 0°–40°W and 5°N–5°S. Earlier results from the eastern equatorial Pacific Ocean showed the opposite trend, with greater Holocene than glacial opal burial, but approximately the same magnitude of difference between glacial and interglacial opal burial. We suggest four (nonexclusive) scenarios to explain the data from both basins: (1) Increased upwelling in the equatorial Atlantic and El Niño–like conditions suppressing upwelling in the eastern equatorial Pacific altered the overall nutrient supply to both basins; (2) Si leaked from the Southern Ocean because of Fe fertilization, but was prevented from upwelling in the equatorial Pacific because of El Niño–like conditions during the LGM; (3) Si leaked from the Southern Ocean because diatom productivity was limited by increased sea ice extent and was again prevented from upwelling in the equatorial Pacific; or (4) changes in ocean circulation related to the decreased input of Agulhas water to the glacial South Atlantic provided excess dissolved Si to the equatorial Atlantic Ocean. A deglacial opal pulse is seen in both the Atlantic and Pacific Oceans. All four scenarios and the presence of the common deglacial opal pulse suggest a common driver in the Southern Ocean.

Silicic acid dynamics in the glacial sub‐Antarctic: Implications for the silicic acid leakage hypothesis

The silicic acid leakage hypothesis (SALH) purports that changes in silicon and nitrogen depletion ratios in the glacial Antarctic created a large pool of unused silicic acid that was transported to lower latitudes in subantarctic mode water (SAMW) where it enhanced diatom productivity lowering atmospheric pCO2. However, increased opal accumulation beneath the sub‐Antarctic during glacial times implies significant consumption of silicic acid in subantarctic surface waters that may have significantly diminished or eliminated Si leakage. To test how nutrient dynamics in the sub‐Antarctic affected the Si(OH)4 content of SAMW during the last glacial period, we produced δ30Si opal records for cores from the subantarctic and subtropical zones of the Indian Ocean spanning the last 50,000 years. Comparison with diatom‐bound δ15N records shows that subantarctic surface waters were enriched in Si relative to N during the last glaciation consistent with the SALH. The record from the subtropics does not show this enrichment in Si during the last glacial period suggesting that subantarctic surface waters were mainly incorporated into SAMW rather than being transported across the Subtropical Front. Isotope mass balance calculations were used to test for Si leakage from the sub‐Antarctic. The results show that silicic acid concentration in SAMW would more than double during the last glaciation if upwelling and northward Ekman drift in the Antarctic were similar to the present‐day circulation. Calculations that assume increased stratification in the glacial Antarctic eliminate Si leakage, but they do not produce the known increase in opal burial in the glacial sub‐Antarctic. Reconciling the isotope data and opal burial records with a highly stratified Antarctic requires the addition of a large local source of silicic acid in the glacial sub‐Antarctic that is inconsistent with present‐day circulation and nutrient distributions. Including such a source in our calculations results in significant opal burial in the sub‐Antarctic, but it does not enhance Si leakage over that occurring in the Holocene. Resolving past changes in the circulation of the Southern Ocean is clearly vital to future tests of the SALH.

Silicic acid leakage from the Southern Ocean: Opposing effects of nutrient uptake and oceanic circulation

Studies invoking the silicic acid leakage hypothesis to explain events of high biogenic opal deposition at low latitudes did not consider variations in formation rate of Southern Ocean intermediate waters. Comparison of δ30Si records from the Southern Ocean to records of biogenic opal accumulation from low‐latitude upwelling systems indicates a strong negative correlation between the silicic acid content of Southern Ocean surface waters and opal deposition at low latitudes during the last 80 kyr BP. This implies that Si leakage was somehow rendered ineffective when the silicic acid content of Southern Ocean waters was high. Comparison of opal accumulation records with records of intermediate water formation rates suggests that lower rates of intermediate water formation offset the higher silicic acid content of these water masses during glacial periods. This indicates that physical processes that control intermediate water transport dominate over biological processes that set the silicic acid content of Antarctic surface waters in determining the magnitude of silicic acid leakage from the Southern Ocean.

Changes in opal flux and the rain ratio during the last 50,000 years in the equatorial Pacific

Abstract Changes in the orgC/CaCO3 ratio in particles sinking from the surface to the deep ocean have the potential to alter the atmospheric pCO2 over the span of a glacial/interglacial cycle. Recent paleoceanographic and modern observational studies suggest that silica is a key factor in the global carbon biogeochemical cycle that can influence the flux ratio, especially at low latitudes, through “silicic acid leakage” [Brzezinski, M., Pride, C., Franck, M., Sigman, D., Sarmiento, J., Matsumoto, K., Gruber, N., Rau, R., Coale, K., 2002. A switch from Si(OH)4 to NO 3 - depletion in the glacial Southern Ocean. Geophysical Research Letters 29, 5]. To test this hypothesis, we reconstruct biogenic fluxes of CaCO3, orgC and Si for three equatorial Pacific cores. We find evidence that a floral shift from a SiO2-based community to a CaCO3-based occurred, starting in mid-marine isotope stage (MIS) 3 (24–59 cal. ka) and declining toward MIS 2 (19–24 cal. ka). This could reflect the connection of the Peru upwelling system to the subantarctic region, and we postulate that excess silica was transported from the subantarctic via the deep Equatorial Undercurrent to the eastern equatorial Pacific. In the eastern equatorial Pacific only, we document a significant decrease in rain ratio starting mid-MIS 3 toward MIS 2. This decrease is concomitant with a significant decrease in silica accumulation rates at the seabed. This pattern is not observed in the Pacific influenced by equatorial divergence and shallow upwelling, where all reconstructed fluxes (CaCO3, orgC, and opal) increase during MIS 2. We conclude that the overall calcium carbonate pump weakened in the EEP under Peru upwelling influence.